1. Field of the Invention
The present invention generally relates to a write-once-read-many (WORM) optical disk, and more particularly to a write-once-read-many optical disk being able to perform high density recording in a wavelength range that is no greater than blue laser wavelength.
2. Description of the Related Art
In recent years and continuing, development of blue laser, which enables ultra-high density recording, is rapidly growing. Furthermore, write-once-read-many type optical disks are being developed along with the rapid development of blue laser.
In the conventional write-once-read-many type optical disk, a laser beam is irradiated to a recording layer formed of an organic material so as to form recording pits in the recording layer (the recording pits mainly formed by change of refractive index created by decomposition/alteration of the organic material). Accordingly, the optical constant and decomposition behavior of the organic material used in the recording layer are important elements for forming satisfactory recording pits.
It is, therefore, necessary to select a material having suitable optical property with respect to blue laser wavelength and decomposition behavior. That is, for the purpose of increasing reflectivity in an unrecorded state, and/or for the purpose of creating a considerable change of refractive index by decomposing the organic material by irradiating laser thereto (thereby obtaining a large degree of modulation), the recording/reproduction wavelength is selected so that it is situated at a bottom of a long wavelength side of a large absorption band.
This is because the bottom toward the long wavelength side of the large absorption band of the organic material is a wavelength area in which a suitable absorption coefficient and a large refractive index can be obtained.
Nevertheless, with respect to blue laser wavelength, there is hardly any organic material providing an optical property with a value equivalent to that of a conventional recording layer. In order to obtain an organic material having an absorption band in the vicinity of the blue laser wavelength, the molecular structure is to be reduced or its conjugated system is to be shortened. This, however, leads to decrease of absorption coefficient (light absorption function), that is, decrease of refractive index.
In other words, although there are many organic materials having absorption bands in the vicinity of the blue laser wavelength range and although it is possible to control absorption coefficient, such organic materials do not have a large refractive index and are unable to attain a sufficient modulation degree (degree of modulation).
Patent Documents 1-5 (corresponding to Japanese Laid-Open Patent Application Nos. 2001-181524, 2001-158865, 2000-343824, 2000-343825, and 2000-335110) are examples showing the organic materials used for blue laser. However, in the embodiments of these documents, merely the solution and spectrum of the thin film are measured. There is no description regarding a recording/reproduction operation in these documents.
Although there are some descriptions regarding recording/reproduction in the embodiments of Patent Documents 6-8 (corresponding to Japanese Laid-Open Patent Application Nos. 11-221964, 11-334206, and 2000-43423), the recording wavelength thereof is 488 nm. Furthermore, the documents do not describe the conditions for recording or the density of recording but merely describe that satisfactory recording pits can be formed.
Although there are some descriptions regarding recording/reproduction in the embodiment of Patent Documents 9 (corresponding to Japanese Laid-Open Patent Application No. 11-58955), the recording wavelength thereof is 430 nm. Furthermore, the document do not describe the conditions for recording or the density of recording but merely describe that satisfactory degree of modulation can be obtained.
Furthermore, although a recording wavelength of 430 nm and a NA of 0.65 are described in the embodiments of Patent Documents 10-19 (corresponding to Japanese Laid-Open Patent Application Nos. 2001-39034, 2000-149320, 2000-113504, 2000-108513, 2000-222772, 2000-218940, 2000-222771, 2000-158818, 2000-280621, and 2000-280620), the recording is conducted with a low recording density (same recording density as DVD) in which the shortest pit is 0.4 μm.
Although a recording/reproduction wavelength of 405-408 nm is described in the embodiment of Patent Document 20 (corresponding to Japanese Laid-Open Patent Application No. 2001-146074), the embodiment does not specifically describe the recording density, but merely describes a low recording density in which 14T-EFM signals are used for recording.
Therefore, the above-described documents show that there is hardly any organic material that is able to provide an optical constant equal to the optical constant desired for a recording layer of a conventional write-only-read-many optical disk, in the vicinity of the 405 nm (center area of the oscillation wavelength of the blue semiconductor laser employed in current practical use).
Furthermore, there is no example which clearly defines the recording conditions in the vicinity of a wavelength of 405 nm and records with a recording density higher than that of DVD.
Furthermore, in a write-only-read-many optical disk employing a conventional organic material, the main absorption band of the organic material is situated in the vicinity of the recording/reproduction wavelength. Therefore, the dependency of the optical constant of the organic material with respect to wavelength becomes greater (thereby causing complex refractive index δn and/or δk to change considerably even when there is only a slight change of wavelength δλ, see FIG. 1). Therefore, this leads to a problem where reflectivity, and recording characteristics, such as recording sensitivity, modulation degree, jitter, and error rate to change considerably when change of recording/reproduction wavelength occurs due to, for example, individual differences amongst lasers or change in ambient temperature.
Meanwhile, from the aspect of reducing the cost and energy for developing pigments, Patent Document 21 (corresponding to Japanese Laid-Open Patent Application No. 2002-74740), describes an optical recording medium having a pigment with a maximum absorption that is situated toward a long wavelength compared to the wavelength of light beam employed for writing. However, the difference between this example and the present invention is described below.
Conventionally, it is typical for a write-only-read-many optical disk to have a High-to-Low recording property. Therefore, it is necessary to select an organic material, such as a pigment capable of providing a complex refractive index that is substantially equal to that of a conventional recording layer. However, since the typical organic material for short wavelength has a small molecular structure, there are few parts to which a substituent having a donor/acceptor effect can be applied. Furthermore, even in a case where the molecular structure is large, the substituent is unable to sufficiently provide the donor/acceptor effect since the conjugated system is short.
Therefore, in a case of a recording/reproduction wavelength area that is no greater than that of a blue laser, it is extremely difficult to obtain a complex refractive index that is substantially equal to that of a red laser wavelength area. Furthermore, even if there is an organic material that is able to provide a complex refractive index that is substantially the same as that of the red laser wavelength area, it is extremely difficult to optimize the complex refractive index of the organic material to the recording/reproduction wavelength.
Furthermore, in a conventional exemplary case of realizing a High-to-Low recording property, the absorption band of the organic material layer is set to be situated toward the short wave side with respect to the recording/reproduction wavelength range, and the recording/reproduction wavelength is set to be situated at the bottom of the absorption band of the organic material layer. However, under these conditions, there is a problem in which the organic material layer requires to be designed in accordance with the selection of the recording/reproduction wavelength and a problem in which the recording/reproduction property considerably changes with respect to the change in the wavelength of the laser during recording/reproduction.